The present invention relates to a process for the manufacture of tubes of fluorinated resin, particularly polytetrafluoroethylene (PTFE), of slight thickness.
The method of the present invention is of the type described in patent FR-A-2,218,987 of the Compagnie Plastic Omnium, and comprises the steps consisting of winding on a mandrel a ribbon of crude fluorinated resin, particularly PTFE, obtained particularly by lubricated extrusion and calendering, the winding of the ribbon being performed in a plurality of layers, bringing the ribbon thus wound on the mandrel to a temperature above the sintering temperature of the fluorinated resin for a sufficient time to produce sintering cooling the tube and the mandrel, and extracting the mandrel situated inside of the tube.
The tubes thus obtained can be used particularly as sheaths for the lining of metal tubes, particularly as described in Patent FR-A-2,292,175 of the Compagnie Plastic Omnium.
The process described above for making tubes, particularly of PTFE, is satisfactory when the thickness of the tubes formed must be on the order of 3 mm or more, particularly for tubes intended to form sheaths resistant to corrosion by transported chemical products.
In this case the extraction of the mandrel at the end of the heating step intended to produce the gelling of the resin poses no particular problem. It suffices in this case to exert traction while holding the end of the tube, after slightly heating the tube to expand it, if necessary.
It is necessary, on the other hand, for certain applications, for example for sheaths mounted in bundles inside of heat exchangers, to make tubes of fluorinated resin, particularly of PTFE, which have a smaller thickness, i.e., much smaller than that of the tubes traditionally manufactured by the above-mentioned process, for example tubes of 1 to 2 mm of thickness, or even tubes of from one to a few tenths of millimeters of thickness.
In this case it is found, when carrying out the process described, that the extraction of the mandrel with respect to the tube proves to be very difficult, even impossible, without damaging the tube.
When the operation of preliminary heating is performed, the tube shrinks strongly, mainly in its diameter, and there is a great tightening stress of the tube on the mandrel, preventing extraction.